This invention relates generally to Electromagnetic Interference (EMI) shielding structures. More specifically, this invention relates to a compact EMI shield structure that also incorporates a heat sink.
Compact, portable computers are becoming increasingly popular among college students, businesspeople, writers, and others who require portable word-processing, e-mail, and computer graphics capabilities. In particular, notebook-sized computers, commonly known as xe2x80x9cnotebook computers,xe2x80x9d are becoming increasingly popular because their small size and low weight make them portable and convenient to use.
Generally, a notebook computer comprises two main sections coupled together by hinges in a clamshell configuration. The first section contains a liquid crystal display (LCD) for displaying information. The LCD screen is disposed in a LCD bezel frame. Typically, the bezel frame is less than about one centimeter in thickness. The second section of the notebook computer comprises a compact computer base section with a keypad area for entering data. The computer base of a notebook computer has a low profile in that its vertical thickness is as thin as possible to minimize the bulk of the notebook computer in its folded configuration. Typically the computer base section of a notebook computer is less than about four centimeters in height and is preferably about two centimeters in height. Typically the computer base section has a width and length comparable to a notebook pad (e.g., preferably less than about 8xc2xd inches by 11 inches). Generally it is desirable to reduce the width and the length of the computer base section as much as possible consistent with a keyboard design that is comfortable for the user to input data with for extended periods of time. Commonly, conventional QWERTY keyboards are used in notebook computers, with the QWERTY keyboard substantially filling the upper surface of the computer base section. However, alternate computer keyboard designs that achieve the function of a QWERTY keyboard in a more space efficient configuration are known to those of ordinary skill in the art.
Notebook computer designers face the challenge of increasing the functionality of a notebook computer while maintaining a low-profile housing. Typically, the centermost portion of the computer base is densely packed with electronic circuits that must fit into a volume that has a vertical height of between two-to-three centimeters. For example, the main motherboard is preferably a double-sided motherboard with electronic chips on both sides of a printed circuit board. In addition to the motherboard, the centermost portion of the computer base also contains support and mounting elements, electrical interconnection elements, and electrical isolation elements. Moreover, the frame of the computer base and the keyboard assembly also consumes part of the vertical height of the computer base.
The central processing unit (CPU) of a high performance notebook computer operates at a high clock rate. Heat is generated at every switching event. Consequently, a high clock rate causes the CPU to generate heat at a rapid rate. This heat must be dissipated to maintain the CPU at an acceptable operating temperature. One solution to dissipating the heat from high performance chips that are used in a conventional desk-top computer is to couple a high performance heat sink to the CPU. However, high performance heat sinks typically have a substantial thickness associated with the finned heat sink and the fan used to blow air over the fins of the heat sink. As an illustrative example, a compact heat sink and fan that is about one centimeter in height would consume a substantial fraction (e.g., about one-third) of the height of the computer base section of a notebook computer that is three centimeters in height. Although some miniaturization of a conventional heat sink and fan is possible, conventional extruded (finned) heat sinks are typically between about five-to-ten millimeters in height while conventional fans are typically a substantial fraction of a centimeter in height. Consequently, use of conventional heat dissipation structures which include a finned heat sink and fan may be inconsistent with a low profile notebook computer base that has sufficient vertical height for a double-sided motherboard and other mounting, support, and electronic interconnection elements.
Electromagnetic interference (EMI) is also a problem in high performance notebook computers. The high clock rate of the CPU of a high performance notebook computer is associated with high-frequency signal components. These high-frequency signal components may generate electromagnetic waves which propagate to other portions of the notebook computer or to neighboring electronic circuits and produce deleterious electromagnetic interference (EMI). An additional EMI shield comprising a conductive enclosure substantially surrounding the CPU is required to reduce the EMI to acceptable levels. According to well-known principles of electromagnetic theory, a conductive enclosure shields, or blocks, the propagation of electromagnetic radiation from an enclosed source. An EMI shield enclosure is commonly shaped as a six-sided box, although it may have other shapes that substantially enclose the EMI source. Some conventional notebook computers utilize five pieces of sheet metal in the computer base section with an additional separate metal cover to form a six-sided EMI shield substantially surrounding the motherboard. However, using sheet metal to form a six-sided EMI shield significantly increases the size and weight of a notebook computer. Alternatively, a substantially six-sided EMI shield enclosure may be formed in a notebook computer by coating the inner walls of the computer base housing with a conductive coating. However, since electronic components must be tightly spaced within a notebook computer, proactive measures must be taken to prevent shorting of electronic circuits to the EMI shield, such as adding additional insulating spacer elements to electronically isolate electronic circuit elements from the conductive walls of the EMI shield. Application of a conductive coating may also have fabrication disadvantages, such as problems associated with applying a high-quality conformal coating that properly adheres to housing surfaces.
One technique to reduce the weight of an EMI shield is to form a bag-like enclosure out of a flexible conductive film instead of sheet metal. Unfortunately, a flexible EMI bag-like enclosure comprised of insulating and conducting layers is largely inconsistent with the use of conventional heat sinks. For example, U.S. Pat. No. 5,436,803 teaches the use of a flexible electrically insulating bag with additional metal fibers embedded in the insulated material of the bag. Similarly, U.S. Pat. No. 5,597,979 teaches the use of a bag-like EMI shield comprising a conductive material either embedded in or laminated on one side of an insulating sheet. A flexible bag-like enclosure with an insulated interior surface provides the advantage that the bag-like enclosure may be slipped around an assembled printed circuit board. A neck or partially open end of a flexible bag-like enclosure may also facilitate making/changing electrical connections to the printed circuit board enclosed by the bag. However, bag-like enclosures are inconsistent with the use of conventional heat sinks. This is primarily because a bag-like EMI enclosure placed around a circuit board would block the flow of air across an interior heat sink. Thus, a bag-like EMI enclosure surrounding an internal circuit board assembly and heat sink is inconsistent with effective cooling of the electronics assembly by heat exchange to the atmosphere.
In addition, a conventional heat sink cannot be effectively thermally coupled to an electronics assembly contained within a flexible EMI enclosure. It is well known in the art of materials science that an electrically insulating layer tends to be a good thermal insulator. Consequently, a bag-enclosure comprised of an outer electrically conductive layer attached to (or embedded in) an electrically insulating layer will tend to create a large thermal resistance between an enclosed electronics assembly and an exterior heat sink. An additional problem with EMI shields comprising bag-like enclosures is that conventional polymers and plastics used to fabricate a flexible electrically insulating film tend to deform and/or melt at common solder bonding process temperatures. Thus, it would be difficult to use a solder process to achieve an effective thermal or electrical coupling between the EMI-shield enclosures of U.S. Pat. Nos. 5,436,803 and 5,597,979 and conventional heat sinks.
The combined size and weight of an EMI shield and heat sink is significant in the context of a notebook computer. Unfortunately, conventional EMI shields cannot be directly combined with conventional heat sinks to achieve a substantial reduction in the vertical height and/or weight of a notebook computer. The electronics package of U.S. Pat. No. 5,175,613 utilizes the conductive surface of a finned heat sink to form an upper surface of an EMI shield surrounding a chip disposed on a single (top) surface of a printed circuit board. The finned heat sink of U.S. Pat. No. 5,175,613 is mechanically secured to the printed circuit board by a bolt or screw. The bolt or screw also electrically couples the heat sink to a ground reference plane disposed in the center of the printed circuit board. The package of U.S. Pat. No. 5,175,613 provides EMI and thermal protection to circuit chips disposed between the conductive surfaces of the heat sink and the ground reference plane of the printed circuit board. However, the package of U.S. Pat. No. 5,175,613 is not designed to address the space and weight constraints of a high performance notebook computer.
One problem with the package of U.S. Pat. No. 5,175,613 is that the use of a finned heat sink typically consumes a significant vertical height and often requires an additional fan element to achieve satisfactory heat dissipation results. For example, finned (extruded) heat sinks suitable for cooling a motherboard assembly typically have a height of five-to-ten millimeters, which is a significant fraction of the available vertical height in the computer base of a notebook computer. Also, conventional finned heat sinks commonly comprise a significant amount of a high conductivity metal, such as copper. Thus, while the combined heat sink/EMI shield eliminates the need for additional EMI shielding components, the total height and weight of the package may still be undesirably large for notebook computer applications.
Another problem with the package of U.S. Pat. No. 5,175,613 is that it is inconsistent with a double-sided motherboard that has chips mounted to both the top and bottom sides of the printed circuit board. U.S. Pat. No. 5,175,613 explicitly teaches that the electronic chips are disposed on only one side of a printed circuit board. This permits the underlying ground reference plane of the printed circuit board to be used to form one surface of an EMI shield surrounding the chips. In order to modify the teachings of U.S. Pat. No. 5,175,613 for a double-sided motherboard, a second heat sink (or other five-sided EMI enclosure comprised of sheet metal) would have to be added around the bottom side of the motherboard to form an EMI shield around the chips disposed on the bottom side of the printed circuit board. Also, additional screw or bolt means would be required to electrically couple the second heat sink to the reference plane of the printed circuit board in order to form an EMI shield. The thickness and weight of the total assembly may be inconsistent with the space and weight constraints of a compact notebook computer.
No previously known notebook computer has addressed the problem of combining the function of a lightweight EMI shield and a compact heat sink in a synergetic manner. However, the problems of achieving effective heat dissipation and EMI shielding in a low profile notebook computer design will become progressively worse as CPU clock rates increase. Also, while notebook computers are one example of an electronic device requiring a compact assembly, other electronic devices, such as high performance palmtop computers, face many of the same thermal dissipation and EMI problems.
What is desired is a low-profile assembly for enclosing electronic circuits in an EMI shield while also providing an efficient heat dissipation function.
The present invention is directed to a structure which combines the functions of an electromagnetic interference (EMI) shield enclosure and heat sink. The present invention generally comprises a heat spreading plate comprised of an electrically conductive material and a receptacle formed from an EMI shield material, wherein the receptacle has a grounding tab electrically coupled to the tray so that the receptacle is electrically coupled to the heat spreading plate by bringing the grounding tab of the receptacle into mechanical contact with the heat spreading plate.
One aspect of the present invention is the use of a bilayer EMI material comprised of an insulating interior layer and a conductive outer layer to form the receptacle. A portion of the EMI material may be formed into a grounding tab with an exposed portion of the conductive outer layer of the EMI material for making electrical contact to the heat spreading plate.
Another aspect of the present invention is the use of spacer layers to achieve a predetermined separation between thermally conductive blocks disposed on the heat spreading plate and chips mounted on a motherboard that is enclosed by the EMI shield. Appropriate selection of the separation distance of the spacer elements permits thermally coupling between the thermally conductive block and the chip while also facilitating a pressure contact between the grounding tabs and the heat spreading plate.
Still another aspect of the present invention is a notebook computer assembly in which the heat spreading plate is used to provide a support or stiffening function for other components of the notebook computer. In one embodiment, a portion of the heat spreading plate is utilized to provide mechanical support for a keyboard.
In still yet another aspect of the present invention, an auxiliary heat sink is incorporated in the liquid crystal display section of the notebook computer and thermally coupled to the thermally conductive blocks. The auxiliary heat sink reduces the heat dissipation requirements of the heat spreading plate, facilitating a compact, low profile notebook computer design consistent with a large thermal load.